The planets in our solar system are roughly divided into two groups: small, rocky worlds like Earth and large gas giants. Before the discovery of the exoplanets, our solar system was believed to be very typical. The light and heat from a star push the gas towards the outer solar system, while heavier dust stays closer to the star. For example, a solar system has nearby rocky planets and distant gas giants. But we now know that planets and star systems are much more diverse.
The most common way to categorize exoplanets is by their mass or size. Jupiter worlds are the largest, then Neptunians, super-earths, earth-sized and lower-earths. Obviously, the greatest interest is in potentially habitable Earth-like worlds that would have similar mass and orbit as our planet. But there is still much we do not understand about other types of planets. For example, super-earths are slightly larger than Earth, but they are terrestrial planets or more gas-like. Because of this, the group is sometimes further divided into those smaller than about 1.6 Earth radii, which are likely rocky, and larger super-earths, often called mini-Neptunes, which are likely to have more in common with gas giants.
Exoplanets by size and temperature. Photo credit: NASA / Ames Research Center / Natalie Batalha / Wendy Stenzel
Since we cannot observe most exoplanets directly, one way to study them is to look at their statistics. For example, there is a statistical gap between the great earths and mini-Neptunes. This 1.6 earth radius gap indicates separate forms of formation. Things are less clear for lower-earths. Planets the size of Mars or Mercury are hard to find, which is part of the reason why there are so few known exoplanets underground. This makes it difficult to study their statistics. However, a new statistical study suggests an interesting origin for these little worlds.
Because the number of confirmed lower earths is so small, the team examined a collection of candidate planets. Observational data suggests they might be planets, but the data isn't strong enough to be certain. They have filtered everything out of more than 4,000 candidate planets, except worlds with short orbital times (less than 16 days) and a size of less than 4 earth radii. That left 280 candidates, which is enough to compile some basic statistics.
Earth-sized worlds could be rare. Photo credit: NASA / Ames Research Center / Daniel Rutter
One of the things they found is that the size distribution for these exoplanets follows a power law distribution. In other words, the statistical number of planets increases by an order of magnitude (or force) as they get smaller. The size of the asteroids in our solar system follows a power law distribution, and we know that the asteroids formed long after the classical planets captured much of the material of the early solar system. Since lower earths follow a similar distribution, it is very likely that they formed later as well.
The authors refer to this two-stage educational process as Generation I (large planets) and Generation II (terrestrial sub-earths). If this idea is correct, it could explain why super-earths are more common than truly earth-sized worlds. If planets like Earth were Generation II, they would be pretty rare. However, it is important to note that the sample used is quite small. While the study is interesting, we need more data before we can draw solid conclusions.
Reference: Yansong Qian and Yanqin Wu. "A distinct population of small planets: lower earths." arXiv preprint arXiv: 2012.02273 (2020).